By generating over 10 of millions metric tons of material, hundreds of billions of dollars of production per year, and being responsible for approximately millions of jobs, plastics and related businesses represent the fourth largest industry in the United States. Unlike other material industries such as steel and aluminum, however, this industry depends almost solely on nonrenewable raw material, mostly imported petroleum. This dependence becomes even more significant as the growth rate of plastics continues to outpace that of all other materials.
Most of the plastic supplied by today's manufacturers ends its life in landfills or incinerators simply because the technology has not been available to recover it economically. The Environmental Protection Agency estimates that the amount of plastic in municipal solid waste grew from less than 1 million metric tons prior to 1960 to over 20 million metric tons by 2000. Take-back and producer-responsibility legislation is becoming increasingly common to help deal with the quantities of plastics being produced.
Durable goods, such as automobiles, appliances and electronics equipment, account for about one-third of the plastics in municipal solid waste. Durable goods are increasingly being collected and partially recycled at the end of their useful lives to avoid disposal costs and potential liabilities, and to recover metals and other marketable raw materials.
The recovery of plastics from durable goods requires a plastic-rich raw material. Automobiles, appliances and electronics generally contain metals. Generally, the metals content is higher than the plastics content (typically plastics content is less than 30%) in these items, so a metal recovery operation must precede plastic recovery. Most metal recovery operations shred equipment in order to cost-effectively liberate metals from whole parts. Large-scale plastic recovery operations must be able to source this plastic-rich raw material from a number of metal recovery operations.
Most plastic parts coming from durable goods streams present unique challenges that are not met by the plastics bottle cleaning and sorting processes developed for curbside feedstocks. The principle practice today for the recovery of highly contaminated scrap is hand-separation done overseas at significant local environmental cost. The challenges in recycling plastics from durable goods include: multiple plastic types, multiple resin grades of plastic (there can be over 50 different grades of one plastic resin type such as ABS); fillers, reinforcements, and pigments; metal; paint and metallic coatings; and highly variable part sizes and shapes.
A grade of plastic is a formulation of plastic material with a particular set of targeted physical characteristics or properties. The particular physical characteristics or properties of a grade are controlled by the chemical composition of the polymers in the grade, the average molecular weights and molecular weight distributions of polymers in the grade, the rubber morphology for impact modified grades, and the group of additives in the grade.
Different grades of a given plastic type are generally compatible. Grades can generally be melt mixed to create a new material with a different property profile.
Different plastic types, on the other hand, cannot generally be melt combined as easily unless the types happen to be compatible. Blending different plastic types such as HIPS and ABS is often avoided except in special situations.
Typical suppliers of plastics-rich feed stocks are metal recyclers or shredders who can process a number of types of durable goods in a single facility. Feedstocks derived from durable goods can therefore be highly variable mixtures of different types of durable goods.
In order to create high value products, the plastic recycling process must be able to separate highly mixed streams on a flake-by-flake basis to achieve high throughput rates of products with acceptable purity. Methods such as separation by density, Density Differential Alteration, froth flotation, color sorting and triboelectrostatic separation (TES), have been used to achieve some purification of the plastics derived from durable goods, as described, for example, in Paul Allen, Development of Hydrocyclones for Use in Plastics Recycling, American Plastics Council, 1999, U.S. Pat. No. 6,238,579, U.S. Pat. No. 6,335,376, U.S. Pat. No. 5,653,867, and U.S. Pat. No. 5,399,433, each of which is incorporated by reference herein. The acceptable purity depends on the primary plastic and contaminants.